When a disc is inserted or a drive is connected to your system, Windows looks in the root directory of the new disc or drive for a file named autorun.inf. If found, Windows executes the instructions in that file.For example, an autorun.inf file on a CD might contain a line that reads open=setup.exe. This tells your computer to launch a setup program as soon as the CD is inserted into the drive.

However convenient this might be, unfortunately, AutoRun also opens a huge door for viruses, Trojan horses, and worms. All it takes is a USB flash drive with an autorun.inf file and an executable in its root. Once inserted, a worm launched in this manner can infect every disk partition it finds, jumping from computer to computer as network users connect to an infected drive.The most recent examples of this are the W32/Sality, W32/Virutas and also the W32/Conficker worm which, in addition to spreading via a vulnerability and network shares, also spreads via USB drives.

The free Panda USB Vaccine allows users to vaccinate their PCs in order to disable AutoRun completely so that no program from any USB/CD/DVD drive (regardless of whether they have been previously vaccinated or not) can auto-execute. This is a really helpful feature as there is no user friendly and easy way of completely disabling AutoRun on a Windows PC.

The free Panda USB Vaccine can be used on individual USB drives to disable its AUTORUN.INF file in order to prevent malware infections from spreading automatically. When applied on a USB drive, the vaccine permanently blocks an innocuous AUTORUN.INF file, preventing it from being read, created, deleted or modified. Once applied it effectively disables Windows from automatically executing any malicious file that might be stored in that particular USB drive. The drive can otherwise be used normally and files (even malware) copied to/from it, but they will be prevented from opening automatically. Panda USB Vaccine currently only works on FAT & FAT32 USB drives. Also keep in mind that USB drives that have been vaccinated cannot be reversed.